Ensayo de resistencia a la flexión

The use of buffer zones as a primary means of removing sediment and other pollutants from runoff is a relatively new practice. Historically, sediment control efforts minimised off-site pollution by reducing upland erosion and surface runoff. Buffer zones, on the other hand, are designed to remove sediment from runoff once it has left the upland area. The major sediment removal mechanisms associated with buffer zones involve changes in flow hydraulics which enhance the opportunity for the infiltration of runoff and fine sediment into the soil, sediment deposition and filtration of sediment by vegetation. For these mechanisms to be effective, it is essential that the surface runoff passes slowly through the buffer to provide sufficient contact time for the removal mechanisms to function.

Infiltration is one of the most significant removal mechanisms affecting buffer zone performance. Infiltration is important since the finer sediment particles enter the soil profile along with infiltrating water and because it decreases surface runoff, thus reducing sediment transport capacity. Since infiltration is one of the more easily quantifiable mechanisms affecting buffer zone performance, many buffer zones have been designed to allow all runoff from a design storm to infiltrate into the buffer zone. This approach results in large land requirements because it ignores other removal mechanisms. Buffer zones also remove sediment through the process of deposition. Buffer zones are usually composed of either dense herbaceous vegetation or forests with sparse understorey and dense surface litter. These surface conditions offer high resistance to shallow overland flow and decrease the velocity of overland flow immediately upslope and within the buffer zone, causing significant reductions in sediment transport capacity. If the transport capacity is less than the incoming sediment load, then the excess sediment may be deposited and trapped. The filtration of sediment by vegetation during overland flow is not as well-understood as the infiltration and deposition processes. Infiltration is probably most significant for clay sized particles while deposition is most significant for silt and larger sized particles. Filtration is significant only with the largest soil particles and aggregates.

Grass buffers

Wilson (1967) conducted one of the first buffer zone sediment trapping studies. He reported optimum distances required to trap sand, silt and clay in flood waters on flat slopes and concluded that buffer length, sediment load, flow rate, slope, grass height and density, and degree of vegetative submergence all affect sediment removal. Neibling and Alberts (1979) used a rainfall simulator on grass plots with a slope of 7% to show that 0.6 to 4.9 m long grass buffers reduced sediment discharge by over 90%. Clay transport was reduced by 37, 78, 82 and 83%, for the 0.6, 1.2, 2.4 and 4.9 m buffers, respectively. Significant deposition of solids was observed just upslope of the leading edge of the buffer zone and 91% of the incoming sediment load was removed within the first 0.6 m of the buffer zone.

The most comprehensive research on sediment transport in grass buffers was conducted at the University of Kentucky (Barfield et al., 1979; Hayeset al., 1979; Tollneret al., 1982). Design equations were developed relating the fraction of sediment trapped in vegetation to the mean flow velocity, flow depth, particle fall velocity, buffer length and the spacing hydraulic radius. High trapping efficiencies were reported as long as the vegetation was not submerged, but trapping efficiency decreased

dramatically at higher runoff rates which inundated the media. The Kentucky researchers, like Neibling and Alberts (1979), observed that the majority of sediment deposition occurred just upslope of the buffer and within the first metre of the buffer, until the upper portions of the buffer were buried in sediment. Subsequent flow of sediment into the buffer resulted in the advance of a wedge-shaped deposit of sediment down through the buffer. The Kentucky researchers did not consider the long- term effectiveness of buffers.

Younget al. (1980) used a rainfall simulator to study the ability of 27.4 m grass buffers with 4% slopes to control pollution from feedlot runoff. Sediment losses were reduced by 66 to 82%. Magette et al.

(1989) used a rainfall simulator on field plots to study the effectiveness of 4.6 and 9.2 m grass buffers in removing nutrients and sediment from agricultural runoff. Sediment losses were reduced 52 and 75% by the 4.6 and 9.2 m buffers, respectively. Buffer zone effectiveness was also reported to decrease with time and with decreasing buffer zone to source area ratio. Dillaha et al. (1989a) used a rainfall simulator to evaluate the effectiveness of grass buffers for sediment and nutrient trapping. Plots were constructed with both shallow uniform flow and concentrated or channelised flow. The 9.1 and 4.6 m buffers with shallow uniform flow removed 87 and 75% of the incoming sediment. Buffers with concentrated flow were much less effective, with percentage reductions averaging 23 to 37% less for sediment.

The effectiveness of existing grass buffer zones in Virginia was qualitatively evaluated by visiting and observing buffers on 18 farms in Virginia over a 13-month period (Dillahaet al., 1989b). Buffers were evaluated by talking with landowners and conservationists and observing site conditions. All the buffers were approximately 6 m in length and were used in combination with cropland. Most were installed for water quality improvement in conjunction with Virginia’s Chesapeake Bay Programme. Buffer performance was generally judged to fall into two categories depending on site topography. In hilly areas, grass buffers were judged ineffective for sediment trapping because drainage usually concentrated in natural drainageways within the fields before reaching the buffers. Flow across these buffers during larger runoff-producing storms (the most significant in terms of sediment loss) was therefore primarily concentrated and the buffers were locally inundated and ineffective. This assessment was confirmed by the fact that little sediment was observed to have accumulated in the majority of the buffers observed. Buffers in these areas, while not effective for trapping sediment, were beneficial because they provided cover in areas adjacent to streams, which are often susceptible to severe localised channel and gully erosion. They also provided a narrow buffer between cropland and streams that reduces aerial drift of fertilisers and pesticides to streams during application. The effects of buffers in trapping sediment deposition during channel overflow and floodplain inundation events was not considered.

In flatter areas, such as the Coastal Plain, buffers appeared to be more effective. Slopes were more uniform and significant portions of runoff entered the buffers as shallow uniform flow. Most sediment was observed to deposit just upslope of, or within the first metre of, the field/buffer interface due to the abrupt increase in vegetation density that slows water and reduces its sediment transport capacity. The coarser sediment deposited at the field edge often formed a berm that blocked further inflow of surface runoff into the buffer zone at this point. Several buffers were observed that had trapped so much sediment that they were higher than the adjacent fields. In these cases, runoff flowed parallel to the buffer until a low point was reached where runoff crossed the buffer as concentrated flow. In this situation, the buffer acted more like a terrace. Flow parallel to the buffer zone also was observed on several farms where mouldboard ploughing was practised. When soil was turn-ploughed away from the buffer, a shallow ditch was formed parallel to the field. If this ditch was not removed later by careful disking, runoff concentrated and flowed parallel to the buffer until it reached a low point and crossed as channel flow. Berms of this type were observed with both forest and grass buffers. In response to this problem, one conservationist required landowners participating in the state buffer strip cost-share programme to construct berms or water bars perpendicular to and at 15 to 30 m intervals along width of the buffers. The water bars minimised flow parallel to the buffer and encouraged more uniform distribution of runoff through the buffer. The water bars, which were made

35

T.A. Dillaha III and S.P. Inamdar

with a tobacco ridger, were broad and shallow enough that they did not interfere with planting and harvesting operations. Water bars of this type would be particularly useful in forest buffers to distribute runoff more evenly across the width of the buffer, rather than allowing channelised flow parallel to the buffer until a developed channel is encountered. Water bars are a common forestry conservation practice used to minimise erosion on logging roads.

Most of the grass buffers observed by Dillahaet al. (1989b) had significant forest buffers between them and receiving waters. In many cases, surface runoff was able to pass through the forest buffer in well- developed channels with little opportunity for sediment deposition. In other cases (lower sloped areas in the Coastal Plain), well-developed channels seemed to disappear as they moved into the forested area and high rates of sediment trapping would be expected. Conclusions drawn from the plot and field observations include (Dillahaet al., 1989a; 1989b):

1. Buffers of reasonable length are effective for sediment removal only if flow is shallow and uniform and if the buffers have not been previously inundated with sediment.

2. The effectiveness of herbaceous buffers for sediment removal appears to decrease with time as sediment accumulates in the buffer and encourages concentrated flow across the buffer.

3. Active maintenance is required for sustainable sediment trapping in herbaceous buffers.

a) At sites with significant flow parallel to the buffer, water bars should be constructed perpendicular to the buffer at 15-30 m intervals to intercept runoff and force it to flow through the buffer before it can concentrate further.

b) To promote vegetative growth and sediment trapping, herbaceous buffers should be mowed and the residue harvested 2-3 times per year. Mowing and harvesting of vegetation will increase vegetation density at ground level, reduce sediment transport and remove nutrients from the system.

c) Caution should be used when applying herbicides to adjacent fields to prevent accidental damage to buffer vegetation.

d) Buffers should not be used for roadways or turn rows because traffic will damage the buffers and may cause concentrated flow problems.

e) Cattle should be excluded from buffers at all times, but especially during periods when soils are moist and buffers are most susceptible to hoof damage.

f) Buffers should be inspected regularly for damage caused by farming operations and should be repaired as soon as possible.

g) Buffers that have accumulated excessive sediment must be ploughed out, disked and graded if necessary and re-seeded, in order to re-establish shallow sheet flow conditions.

h) Extreme care must be taken during tillage operations to avoid reducing the length of the buffer. If mouldboard ploughing is practised, the last plough pass should turn soil toward the buffer and the disturbed area next to the buffer should be carefully disked to minimise gully formation and concentrated flow parallel to the buffer.

4. Most on-farm herbaceous buffers observed were judged to be ineffective for sediment removal because most flow tended to accumulate in natural drainageways before reaching the buffer zone. This was more of a problem in hilly areas and less of a problem in flatter areas such as the coastal plain.

5. Buffers should be installed on the contour as much as possible to promote shallow uniform flow across the buffer.

6. Large fields with significant natural drainageways or grassed waterways are acceptable for buffers only if buffers are installed on both sides of internal field drainageways. This will trap sediment before it can enter the drainageways.

7. If slope-lengths are not excessive and buffers are on the contour, up and down slope tillage (perpendicular to the buffer) is preferred over contour tillage because it will distribute runoff more uniformly along the length of the buffer. Unfortunately, it is difficult to till up and downslope without damaging the buffer because of the necessity of using it as a turn row.

Dillaha et al. (1989a) concluded that the effectiveness of experimental buffer zones with shallow uniform flow, which are typically used by researchers in short-term studies, should not be used as a direct indicator of real world buffer zone effectiveness because of long-term sediment accumulation and concentrated flow problems previously discussed. Concentrated flow effects under real agricultural conditions were estimated to be orders of magnitude greater than those encountered during experimental plot studies.

Natural forest buffers

Cooperet al. (1987) used 137_{Cs data and sediment mapping techniques to estimate sediment trapping in}

a forested buffer zone receiving cropland runoff in a Coastal Plain watershed. The riparian buffer was found to remove 84 to 90% of the sediment eroded from the cropland. Sand and coarse sediments were deposited at the forest edge, while silt- and clay-sized particles were trapped deeper in the buffer. Cooperet al. suggest that buffer length should increase as stream order increases because the opportunity for sediment deposition decreases and transport capacity increases as stream order increases. Smith (1989) reported that excluding cattle from a 10 to 13 m length riparian pasture reduced sediment loading to the receiving stream by 87%. Castelle et al. (1994) reported that the relationship between buffer length and sediment removal was non-linear in that disproportionately long buffers are required for increased sediment removal. For example, increasing sediment removal from 90 to 95% on a 2% slope would require buffer length to be doubled from 30.5 to 61 m. In a literature review, Gilliam (1994) reported that riparian buffers are the most important factor reducing sediment loadings to receiving waters, with sediment trapping efficiencies of 85 to 90% commonly reported in Coastal Plain regions. In another literature synthesis, Lowrance et al. (1995) reported similar trapping efficiencies, 80 to 90%, in forested Coastal Plain buffer zones. Sediment trapping efficiencies in other physiographic regions have not been as well investigated and are probably lower than those reported for the Coastal Plain because of steeper slopes and channelised flow effects. Lowrance et al. (1995) reported that buffer zones are the most effective with first and lower order streams because of the greater potential for interaction between upland runoff and the riparian zone in lower order streams. For ephemeral and first order streams, the potential impact of buffer zones in trapping sediment is directly proportional to the proportion of surface runoff from the contributing area that moves through the buffer zone as shallow sheet flow. Smithet al. (1993) suggest that buffers are very important for ephemeral channels because they may be greater sources of NPS sediment loads than perennial channels because of their abundance and lower vegetative cover. For second- order and larger streams, sediment reduction will be based on the proportion of surface runoff from the upslope contributing area that flows through the buffer and the proportion of the surface runoff that enters the riparian area through upslope lower order streams. Clearly, as stream order increases, the impact a buffer zone along a particular stream reach can have on the reduction in overall load within that reach is reduced (Lowranceet al., 1995). On a watershed basis, the higher the proportion of streamflow originating from relatively short flow-paths to small streams protected by buffer zones, the greater the potential impact of buffer zones. Similarly, the higher the drainage density of a watershed, the greater the potential benefits of buffer zones.

Zoned riparian buffers

In response to the need for the development of guidelines for the design and management of forest buffer zones, Welsch (1991) suggested that buffer zones should consist of three zones (Figure 1).

37

T.A. Dillaha III and S.P. Inamdar

Zone 1 is a permanent and undisturbed forested zone immediately adjacent to the stream. Zone 2 is a managed forest zone, just upslope of Zone 1, in which timber is periodically harvested. Zone 3 is a managed herbaceous strip, usually grasses, just upslope of Zone 2 that is used to control runoff. The three-zone forest buffers are specified for habitat and water quality protection of waterbodies adjacent to cropland, pastures and urban areas that are sources of diffuse pollution. Applicable waterbodies included perennial and intermittent stream, lakes, ponds, wetlands and groundwater recharge areas. Required lengths for each of the zones were not presented.

Figure 1. Three zoned riparian buffer zone system (Lowrance et al., 1995)

The principal purpose of the unmanaged forest zone (Zone 1) is aquatic habitat protection and streambank protection. The primary purposes of the managed forest zone (Zone 2) are removal of sediment from overland flow and nutrients from overland and subsurface flow. Zone 3, the managed runoff control zone, has two principal purposes. First, because it is composed of close growing herbaceous vegetation, usually grass, it offers high resistance to overland flow and is thus an effective sediment deposition area under shallow flow conditions. Secondly, because it is a managed and possibly constructed zone, it can be designed to minimise the movement of runoff into Zone 2 as concentrated flow. Under shallow, uniform flow conditions (minimal concentrated flow), Zone 3 will be responsible for most of the sediment trapping in the three zone buffer. Once Zone 3 is inundated with sediment, and/or flow into Zone 2 is no longer shallow and uniform, Zone 3 will have to be reconstructed and trapped sediment moved back up into the contributing source area. Because of known problems with channelised flow through buffer zones, several researchers (Welsch, 1991; Franklin et al., 1992) have suggested that Zone 3 could also incorporate engineered level-lipped spreaders to distributed runoff across a wider portion of Zone 2.

Lowrance et al. (1995) provided a comprehensive research synthesis on the likely effectiveness of Welsch’s three zone forest buffer system in the Chesapeake Bay drainage basin. They discussed the likely effectiveness of forest buffers for habitat protection and removal of sediment and nutrients in the three different physiographic regions of the Chesapeake Bay. Most of their report deals with the Coastal Plain region where conditions are ideal for buffer zones (widespread existing forested riparian zones, lower slopes, sandy soils and low upland to riparian land ratios, typically 2:1 to 3:1) and where the most detailed research on buffer zones for water quality protection in the US has been conducted. Research on riparian zone effectiveness for sediment removal in the Piedmont and the Valley and

Ridge physiographic provinces was reported to be very limited and it was difficult to make quantitative estimates of buffer zone effectiveness in these areas. Sediment removal by buffer zones in the Piedmont was assumed to be similar but somewhat less than that in the Coastal Plain due to higher slopes and greater channelisation of flow through the buffers. Sediment trapping is assumed lower still in the Valley and Ridge because of higher slopes and increased channelisation but trapping can still be significant if the runoff control zone (Zone 3) is well-maintained and flow channelisation is